1. Technical Field
This invention relates to a magnetic carrier head for chemical mechanical polishing, and more particularly to a carrier head attached by magnetic forces to a chemical mechanical polishing apparatus.
2. Background Information
Integrated circuits typically are formed on semiconductive substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive, or insulative layers. After each layer is deposited, the layer is etched to create circuitry features. As a series of layers is sequentially deposited and etched, the outer surface of the substrate, i.e., the exposed surface of the of the top layer on the substrate, becomes successively more non-planar as a result of peaks and valleys which are formed by the selective etching. This non-planar outer surface presents a problem for the integrated circuit manufacturer. If the outer surface of the substrate is non-planar, then a photoresist layer placed thereon also is non-planar.
A photoresist layer may be placed on the substrate as a step of an etching process. The photoresist layer typically is patterned by a photolithographic apparatus that focuses a light image onto the photoresist layer. If the outer surface of the substrate is sufficiently non-planar, then the maximum height difference between the peaks and valleys of the outer surface may exceed the depth of focus of the imaging apparatus. Consequently, properly focusing the light image onto the entire outer surface becomes difficult if not impossible.
Designing new photolithographic devices having an improved depth of focus may be prohibitively expensive. In addition, as the feature size used in integrated circuits becomes smaller, shorter wavelengths of light must be used, resulting in further reduction of the available depth of focus. Therefore, a need exists to planarize (i.e., make smooth) the substrate surface periodically to provide a planar layer surface.
Chemical mechanical polishing (CMP) is one method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is then placed against a rotating polishing pad. The carrier head applies a controllable load, i.e., pressure, on the substrate to push it against the polishing pad. In addition, the carrier head may rotate to provide additional motion between the substrate and polishing surface.
A polishing slurry, including an abrasive and at least one chemically-reactive agent, may be distributed over the polishing pad to provide an abrasive chemical solution at the interface between the pad and substrate. CMP is a fairly complex process. In a CMP process, the reactive agent in the slurry reacts with the outer surface of the substrate to form reactive sites. The interaction of the polishing pad and abrasive particles with reactive sites on the substrate results in polishing.
The carrier head performs several mechanical functions in the CMP process. Generally, the carrier head holds the substrate against the polishing pad, evenly distributes a downward pressure across the back surface of the substrate, transfers torque from the drive shaft to the substrate, and ensures that the substrate does not slip out from beneath the carrier head during polishing operations. If the carrier head consistently fails to perform any of these functions, it often must be repaired. A first step in many repair procedures is the removal of the carrier head from the rest of the system.